Microporous polyethylene membranes are widely used in separators for lithium batteries, etc., electrolytic capacitor separators, various filters, etc. When the microporous polyethylene membranes are used as battery separators, their performance largely affects the performance, productivity and safety of batteries. Accordingly, the microporous polyethylene membranes are required to have excellent permeability, mechanical properties, heat shrinkage resistance, shutdown properties, meltdown properties, etc. For instance, when microporous polyethylene membranes having low mechanical strength are used as battery separators, battery voltage soon becomes low.
To improve the properties of microporous polyethylene membranes, the optimization of starting material compositions, stretching conditions, heat treatment conditions, etc. have been proposed. WO 97/23554 discloses a microporous membrane having high short-circuiting resistance (shutdown properties), which is made of high-density polyethylene or linear ethylene copolymer having 2 or more terminal vinyl groups per 10,000 carbon atoms  when measured by infrared spectroscopy.
Recently gaining importance as separator characteristics are not only permeability, mechanical strength, heat shrinkage resistance and thermal properties (shutdown properties and meltdown properties), but also battery life properties such as cyclability, battery storability such as oxidation resistance, and battery productivity such as electrolytic solution absorbability. To obtain excellent cyclability and electrolytic solution absorbability, microporous membranes should have excellent permeability and mechanical strength. To obtain excellent safety properties, microporous membranes should have excellent shutdown properties and heat shrinkage resistance. However, microporous membranes for battery separators having well-balanced permeability, mechanical strength, heat shrinkage resistance, shutdown properties, meltdown properties and oxidation resistance have not actually been obtained yet.